1. Field of Invention
Apparatuses and methods consistent with the present invention relate to a projection type image display apparatus employing an optical system provided with a mirror, and more particularly to a projection type image display apparatus having an improved structure capable of keeping performance of an optical system consistent even though surrounding environment changes or use time elapses.
2. Description of the Related Art
In general, a projection type image display apparatus uses light emitted from a light source to focus an image formed by a display device on a screen, thereby displaying the image. To this end, the projection type image display apparatus includes an illumination unit for illumination, the display device for forming an image, and an optical system for projecting an image formed by the display device.
As an example of the optical system, there is a mirror-type optical system including a plurality of mirrors. As compared with a lens-type optical system including a plurality of lenses, the mirror-type optical system has an advantage of little optical aberration, thereby providing an image of high picture quality. However, the mirror-type optical system is more susceptible to changes in the surrounding environment than the lens-type optical system, so that optical performance of the mirror-type optical system may vary largely as the surrounding environment changes.
FIG. 1 is a schematic cross-section view of a conventional projection type image display apparatus employing a mirror-type optical system.
Referring to FIG. 1, the conventional projection type image display apparatus includes a cabinet 1 having an inside space, a screen 3 provided in the front of the cabinet 1, an illumination unit (not shown), a display device 5 to form an image, and an optical system 10 to project an image formed by the display device 5 on the screen 3.
The optical system 10 is placed between the display device 5 and the screen 3, and enlarges and projects an image generated by the display device 5 to the screen 3. For this, the optical system 10 includes a plurality of mirrors M1, M2, M3 and M4, a support structure 11 placed inside the cabinet 1 and supporting each of the mirrors M1, M2, M3 and M4, and a partitioning structure 15 partitioning the inside space of the cabinet 1 into a plurality of spaces.
The partitioning structure 15 prevents air and heat from flowing between the partitioned spaces. Referring to FIG. 1, the partitioning structure 15 partitions the inside space of the cabinet 1 into first through third spaces I, II and III.
In the first space I are provided the illumination unit (not shown); the display device 5; a reflecting surface of the first mirror M1; and the second, third and fourth mirrors M2, M3 and M4 constituting the optical system 10. Further, an inlet 1a for taking in exterior air and an outlet 1b for exhausting interior air are formed in a predetermined region of the cabinet 1 which belongs to the first space I. Also, a fan 7 is provided in the outlet 1b. 
In the second space II is provided a rear surface of the first mirror M1. Further, a ventilation hole 1c is formed in a predetermined region of the cabinet 1 which belongs to the second space II, and ventilates the second space II.
The third space III is formed by the partitioning structure 15, a window 20 through which an image reflected from the fourth mirror M4 is transmitted, the cabinet 1, and the screen 3. Also, a reflective mirror 9 is provided in an interior top side of the third space III and reflects light emitted from the optical system 10 to the screen 3.
As described above, since the inside of the cabinet 1 is partitioned into the first through third spaces I, II and III, the first through third spaces I, II and III may have different environments. Thus, the second through fourth mirrors M2, M3 and M4 placed within the first space I are different in thermal expansion from the first mirror M1 having the rear surface which is in the second space II. Consequently, the performance of the optical system deteriorates as surrounding temperature changes or use time elapses.